Bacteriophage Viruses
Shared 2/18/2026•4 views
### Introduction to Bacteriophages - **Definition:** Viruses that infect and replicate within bacteria. - **Etymology:** "Bacteria" and Greek "phagein" (to devour). - **Abundance:** Most abundant biological entities on Earth. - **Diversity:** Highly diverse in morphology, genome, and infection strategies. ### Structure of Bacteriophages - **Head (Capsid):** Protein shell enclosing the genetic material. - **Shapes:** Icosahedral (most common), filamentous, prolate. - **Genetic Material:** Can be DNA (dsDNA, ssDNA) or RNA (dsRNA, ssRNA). dsDNA is most common. - **Tail:** Varies greatly among phages. - **Contractile:** Myoviridae (e.g., T4 phage). - **Non-contractile:** Siphoviridae (e.g., Lambda phage). - **Short/Absent:** Podoviridae (e.g., T7 phage), filamentous phages. - **Base Plate & Tail Fibers:** Mediate attachment to bacterial surface receptors. ### Replication Cycles #### Lytic Cycle - **Attachment:** Phage binds to specific receptors on bacterial cell wall. - **Penetration:** Genetic material injected into the host cell. - **Biosynthesis:** Phage genes hijack host machinery to produce phage components. - **Maturation:** New phage particles assembled. - **Lysis:** Host cell ruptures, releasing new phages. - **Outcome:** Kills the host bacterium. #### Lysogenic Cycle - **Attachment & Penetration:** Similar to lytic cycle. - **Integration:** Phage DNA (prophage) integrates into the host bacterial chromosome. - **Replication:** Prophage replicates passively with host DNA. - **Induction:** Under stress (e.g., UV radiation), prophage excises from host chromosome. - **Lytic Cycle:** Enters lytic cycle, leading to cell lysis. - **Outcome:** Host bacterium survives and carries the prophage. ### Genome Types - **dsDNA:** Double-stranded DNA (most common, e.g., T4, Lambda). - **ssDNA:** Single-stranded DNA (e.g., M13). - **dsRNA:** Double-stranded RNA (rare). - **ssRNA:** Single-stranded RNA (e.g., MS2). ### Applications of Bacteriophages - **Phage Therapy:** Use of phages to treat bacterial infections. - **Advantages:** Specificity, self-replicating, effective against antibiotic-resistant bacteria. - **Challenges:** Regulatory hurdles, immune response, narrow host range. - **Molecular Biology Tools:** - **Cloning Vectors:** Lambda phage, M13 phage used for gene cloning. - **Phage Display:** Technique to display peptides or proteins on the phage surface for screening. - **Gene Editing:** CRISPR-Cas systems in bacteria often originate from phage defense mechanisms. - **Food Safety:** Used to control bacterial contamination in food products. - **Biocontrol:** Control of plant pathogens. ### Bacterial Defense Mechanisms Against Phages - **Adsorption Inhibition:** Modify surface receptors to prevent phage attachment. - **Restriction-Modification Systems:** Enzymes that cut foreign DNA (restriction enzymes) and methylate host DNA to protect it. - **CRISPR-Cas System:** Adaptive immune system that remembers and degrades foreign nucleic acids. - **Abortive Infection:** Infected cell commits suicide to prevent phage spread.
